Nothing exciting ever happens to boring Dex at his boring home or at boring school. He oversleeps (again!), he daydreams while his science teacher Mr. McFur babbles on about his lab rat Princess Pretty Fabulous (Pretty for short), he gets harassed by Millicent (the neighborhood meanie): boring, boring, ultra-boring! Then one day, a mystery man convinces Mr. McFur to feed Pretty radioactive gamma broccoli, which turns the rat into a rabid rodent the size of a hippo and launches Dex into an underground world of kid spies and rat gas power.

Suddenly Dex's life doesn't seem so boring anymore! But who was that mystery man? What does he want? And most importantly, can a boring Dex shed the boring to become the most unlikely hero in spy history?